A microwave treatment apparatus heats a target object (e.g. food) placed in a heating chamber with a microwave that is generated by a magnetron (i.e. a typical microwave generator) and then supplied to the heating chamber through a waveguide.
Nevertheless an electric field distribution generated in the heating chamber by the microwave supplied is not always uniform. A conventional apparatus uses a motor for rotating a turntable so that a target object can rotate within a heating chamber in order to be heated uniformly. Here is another conventional apparatus that employs a motor for rotating a rotary antenna, thereby agitating the microwave before the microwave is supplied into a heating chamber in order to heat a target object uniformly.
On the other hand, a method for uniformly heating a target object is proposed. This method uses a circularly polarized wave or an elliptically polarized wave, of which polarization plane rotates with the lapse of time. Generation of the circularly polarized wave or the elliptically polarized wave needs a pair of exciting means, of which exciting directions cross each other, for generating a pair of excitations where a phase difference is formed.
FIG. 12 shows an electric current running on a plane of a waveguide in the conventional microwave treatment apparatus. As FIG. 12 shows, rectangular waveguide 100, through which a microwave propagates in TE10 mode, has a cross section that intersects with the longer direction (i.e. the propagating direction of the microwave) at right angles. This cross section forms a rectangle. Wave guide 100 includes narrow plane 102 and wide plane 103.
In such waveguide 100, in the case of forming an opening in cross section 101 vertical to the propagating direction of the microwave, electric field 104 is generated along the same direction within waveguide 100, so that excitation in uniaxial direction is generated. In the case of forming the opening in narrow plane 102, electric current 105 flows along the same direction in narrow plane 102, so that excitation in a uniaxial direction is generated.
Nevertheless, in the case of forming the opening in wide plane 103, electric current 105 flows in various directions depending on a place in wide plane 103, so that excitation in biaxial directions is generated.
Based on the foregoing reason, the opening should be formed in wide plane 103 in order to generate a circularly polarized wave, which is generated by a pair of exciting means of which exciting directions cross each other.
Propagation of the microwave causes an exciting position to move with a lapse of time, so that, for instance, two openings are formed in combination with each other for generating the circularly polarized wave.
FIG. 13A and FIG. 13B schematically illustrate changes in status of generating the circularly polarized wave at opening 107. Opening 107 is shaped like a cross-slot (i.e. two rectangular slots cross each other at right angles) for generating the circularly polarized wave.
FIG. 13A and FIG. 13B show propagating direction 109 of the microwave and a rotating direction of the circularly polarized wave generated at opening 107. FIG. 13A shows the propagating direction of the microwave from the top of the paper toward the bottom of the paper, and, contrary to FIG. 13A, FIG. 13B shows the propagating direction of the microwave from the bottom of the paper toward the top of the paper.
In FIG. 13A, propagating direction 109 in waveguide 100 is directed downward of the paper. Magnetic field 108 generated by the microwave moves downward with a lapse of time.
As FIG. 13A shows, at time to, magnetic field 108 excites a first rectangular slot of opening 107 in exciting direction 110a. At time t1, namely, after a lapse of a given time, magnetic field 108 moves downward, and a second slot of opening 107 is excited in exciting direction 110b. At time t2 and time t3, exciting directions 110c and 110d are changed in turn as illustrated in FIG. 13A, so that the circularly polarized wave that rotates anti-clockwise is generated.
As FIG. 13B shows, propagating direction 109 within waveguide 100 is directed upward on the paper. Magnetic field 108 generated by the microwave moves upward on the paper with a lapse of time. A time lapse from time TO to time t3 causes exciting directions 110a, 110b, 110c, and 110d at opening 107 to change as shown in FIG. 13B, so that the circularly polarized wave that rotates clockwise, which is reversal to what is shown in FIG. 13A, is generated. As discussed above, the circularly polarized wave or the wave rotating in a reversal direction is generated in response to propagating direction 109 within waveguide 100.
FIG. 14 is a schematic plan view of a waveguide, which generates a circularly polarized wave, of a conventional microwave treatment apparatus disclosed in patent literature 1. FIG. 15 is a schematic perspective view of a waveguide, which generates a circularly polarized wave, of another conventional microwave treatment apparatus disclosed in patent literature 2
As FIG. 14 shows, patent literature 1 discloses a structure in which opening 107 is disposed on waveguide 106a. This opening is formed of two rectangular slots crossing each other vertically. As FIG. 15 shows, patent literature 2 discloses a structure in which openings 107a and 107b are disposed in a wide plane of waveguide 106b. These openings 107a and 107b do not cross each other, but disposed vertically to each other.